(1) Field of the Invention
This invention relates to a magnetic sensor utilizing a phenomenon in which a plane of polarization of light traveling through a Faraday element rotates in proportion to the strength of a magnetic field. More particularly, the invention relates to a technique, used in measuring a remote magnetic field, for eliminating influences of variations in the plane of polarization caused by disturbances acting on the light transmitted through optical fibers over a long distance.
(2) Description of the Related Art
A conventional magnetic sensor of this type employs a Faraday element for compactness and high sensitivity. Specifically, the strength of a magnetic field is detected by determining a rotating angle of a plane of polarization applied when polarized light passes through the Faraday element.
Generally, it is adequate to transmit light from a light source by using an optical fiber (single mode optical fiber) and pass the light through a Faraday element with no variations occurring with the plane of polarization. However, the light transmitted through the optical fiber undergoes variations in polarization caused, for example, by phase differences due to disturbances such as environmental conditions encountered during transmission. In such a case, the following techniques are employed to eliminate influences of the variations in polarization due to disturbances and the like.
In a first technique, a polarizer is disposed between the outlet of the optical fiber and the Faraday element to allow only linearly polarized light having a plane of polarization in a particular orientation to pass through the Faraday element.
In a second technique, an optical fiber that maintains a plane of polarization is employed so that light from a light source does not easily undergo variations in the plane of polarization due to disturbances occurring during transmission. As shown in FIG. 1, for example, in order that rays of light R be transmitted with a plane of polarization maintained perpendicular to X-axis, materials 21 are arranged symmetrically across Y-axis to exert a fixed pressure.
However, the conventional constructions noted above have the following drawbacks.
In the first technique having a polarizer disposed between the outlet of the optical fiber and the Faraday element, incident linearly polarized light changes into elliptically polarized light. This is because the light from the light source undergoes indefinite variations in the plane of polarization during transmission through the optical fiber, under the influence of disturbances due to environmental conditions such as temperature and pressure. It is therefore impossible to determine a transmission angle of the polarizer as desired. That is, where the light transmitted through the optical fiber becomes linearly polarized light with a plane of polarization at right angles (90xc2x0) to the polarizer, for example, the light is totally blocked by the polarizer and does not enter the Faraday element.
In the second technique employing an optical fiber that maintains a plane of polarization, where the optical fiber is used over a long distance, a discrepancy of polarization occurs from an initial misalignment (Y-axis in FIG. 1) between the plane of polarization maintaining optical fiber and the plane of polarization of incident light. The discrepancy brings about variations in the plane of polarization due to environmental conditions occurring during transmission. The plane of polarization maintaining optical fiber produces crosstalk which is an intrinsic property thereof. This results in extra polarized components precluding accurate detection results.
With detection results acquired under the foregoing conditions, it is difficult to determine whether the rotation of the plane of polarization is due to a magnetic field or to disturbances such as changes in environmental conditions as noted above.
This invention has been made having regard to the state of the art noted above, and its object is to provide a magnetic sensor of high sensitivity for eliminating influences of disturbance.
The above object is fulfilled, according to this invention, by a magnetic sensor utilizing a phenomenon in which a plane of polarization of light traveling through a Faraday element rotates in proportion to the strength of a magnetic field, the magnetic sensor comprising:
a light output device;
a light branching device connected to the light output device through a first light transmitting device;
a sensor head connected to the light branching device through a second light transmitting device;
a light detecting device connected to the light branching device through a third light transmitting device; and
a computing device for receiving detected signals from the light detecting device;
the sensor head including an optical device, a first birefringent plate, a first Faraday element and a reflecting device arranged in series from an end of the sensor head connected to the second light transmitting device;
the light output device outputting light;
the light branching device receiving the light transmitted from the light output device through the first light transmitting device, and emitting the light to the second light transmitting device;
the optical device of the sensor head converting the light transmitted from the light branching device through the second light transmitting device into parallel light;
the first birefringent plate of the sensor head separating the parallel light received from the optical device into two polarized rays having planes of polarization orthogonal to each other with respect to an optical axis of the first birefringent plate;
the first Faraday element of the sensor head transmitting the two polarized rays from the first birefringent plate, and converting the strength of a magnetic field to be detected into a rotating angle of the planes of polarization of the two polarized rays;
the reflecting device of the sensor head reflecting the two polarized rays transmitted through the first Faraday element, back into the first Faraday element, such that each of the two polarized rays reciprocates along the same optical path;
the first birefringent plate of the sensor head separating each of the two polarized rays returned from the first Faraday element into two polarized rays (four polarized rays in total) orthogonal to each other and having an amplitude level corresponding to the rotating angle of the respective polarized rays;
the optical device of the sensor head selectively transmitting two orthogonal polarized rays returning along optical paths substantially the same as incidence optical paths, among the four polarized rays emitted from the first birefringent plate;
the light branching device branching the two polarized rays transmitted from the optical device through the second light transmitting device, to the third light transmitting device:
the light detecting device detecting light intensities of the two polarized rays transmitted from the light branching device through the third light transmitting device; and
the computing device deriving the strength of the magnetic field from the light intensities detected by the light detecting device.
Specifically, linear light outputted from the light output device may become elliptically polarized light indefinite in both axis and ellipticity due to disturbances occurring in the course of transmission to the sensor head. Such polarized light is passed through the first birefringent plate disposed in the sensor head, whereby the polarized light may be used as separated into two polarized rays orthogonal to each other based on the crystallographic axis of the birefringent plate, while retaining a total energy of light intensity, regardless of rotation of the planes of polarization.
The two polarized rays have the planes of polarization rotated by magnetic field strength in the course of reciprocation through the first Faraday element. These two polarized rays are transmitted through the first birefringent plate again, where each of the two polarized rays is separated into two polarized rays orthogonal to each other and having an amplitude level corresponding to the rotating angle of the respective planes of polarization. The polarized rays not traveling along the same optical paths as the incidence optical paths are omitted by the optical device. That is, the light intensities of the two polarized rays emitted from the optical device are variable with the rotating angle of the planes of polarization of the polarized rays emitted from the first Faraday element. The light intensities of the two polarized rays are detected by the light detecting device, and are put to an arithmetic operation by the computing device, thereby obtaining detection results free from the influences of variations in the planes of polarization due to disturbances.
Since the light is transmitted forward and backward through the first Faraday element, the sensitivity of the element is improved.
Further, since the second light transmitting device is shared by the light entering the sensor head and the light exiting the sensor head, the apparatus may be formed compact.
The foregoing magnetic sensor in the first aspect of the invention may further comprise an element disposed between the first birefringent plate and the first Faraday element for giving a predetermined rotating angle to the plane of polarization of each of the two polarized rays separated by the first birefringent plate.
According to the above magnetic sensor, light that is to be detected without influences of a disturbance or magnetic field is passed through the element disposed downstream of the first birefringent plate, to apply a predetermined rotating angle to the planes of polarization beforehand. As a result, the light intensities detected by the light detecting device are varied according to the direction of the magentic field to be measured.
That is, actual detection results may be compared with a reference provided by the light intensity detected when the magentic field to be measured is xe2x80x9c0xe2x80x9d, whereby the direction as well as the strength of the magnetic field may be detected.
Preferably, the above element is a Faraday element having a predetermined magnetic field applied thereto beforehand.
The magnetic sensor in the first aspect of the invention may further comprise an aperture plate disposed between the optical device and the first birefringent plate and defining a pinhole for passing the two polarized rays to be detected.
With the above construction, the pinhole in the plate disposed between the optical device and the first birefringent plate passes only two polarized rays to be detected. That is, where the first birefringent plate is formed thin to make the sensor head compact, the polarized rays transmitted through the first birefringent plate have small refractive indexes (angles), making it difficult to select and output two polarized rays to be detected. However, by using the pinhole, only two polarized rays to be detected may be selected and outputted.
The magnetic sensor in the first aspect of the invention may further comprise a second optical device disposed between the first birefringent plate and the first Faraday element;
the second optical device refracting two incident polarized rays, and emitting the polarized rays to the first Faraday element, such that the polarized rays intersect each other in the first Faraday element;
the reflecting device reflecting the two polarized rays emitted from the first Faraday element, such that the polarized rays swap optical paths, each returning along an optical path substantially the same as an incidence optical path of the other.
The two polarized rays incident on the second optical device are refracted and emitted toward the first Faraday element. The two polarized rays intersect each other in the first Faraday element. The two polarized rays emitted from the first Faraday element are reflected by the reflecting device, each returning along an optical path substantially the same as an incidence optical path of the other.
Thus, each of the two polarized rays travels through the Faraday element, backward along substantially the same optical path as the forward optical path of the other. Even where the first Faraday element has position dependence for crystal sensitivity, the two polarized rays are given the same level of extra error in rotating angle of the planes of polarization. That is, there is no need to consider measurement errors even though variations may occur in intensity ratio between the two polarized rays due to disturbances acting on the light transmitting device.
The above reflecting device may be attached to or disposed adjacent a light output end of the first Faraday element. Preferably, the reflecting device is a reflecting mirror, or a rectangular prism with reflection coatings applied to polarized light output slant sides thereof.
The magnetic sensor in the first aspect of the invention may further comprise a second optical device disposed between the first Faraday element and the reflecting device disposed adjacent the first Faraday element;
the second optical device refracting two incident polarized rays from the first Faraday element, and emitting the two polarized rays to the reflecting device, such that the two polarized rays intersect each other before reaching the reflecting device;
the reflecting device reflecting the two polarized rays, such that the polarized rays swap optical paths, each returning along an optical path substantially the same as an incidence optical path of the other.
With the above construction, one of the polarized rays refracted by the second optical device and reflected by the reflecting device returns along an optical path substantially the same as an incidence optical path of the other polarized ray, and the other polarized ray returns along an optical path substantially the same as an incidence optical path of the one polarized ray. Thus, where the first Faraday element has position dependence for crystal sensitivity, the two polarized rays are influenced by the same error in rotating angle of the planes of polarization. That is, there is no need to consider measurement errors due to variations in intensity ratio between the two polarized rays.
In the above magnetic sensor, the first Faraday element may be in form of two separate Faraday elements arranged parallel to each other in a traveling direction of the two polarized rays, such that the two polarized rays separated by the first birefringent plate are transmitted through the separate Faraday elements, respectively.
With the two separate Faraday elements arranged parallel to each other in the traveling direction of the two polarized rays, crystals of small sectional area may be used to realize high Faraday effect. The sensor head may also be formed compact.
The magnetic sensor in the first aspect of the invention may further comprise a second optical device disposed between the first birefringent plate and the first Faraday element;
the second Faraday element transmitting and emitting one of the two polarized rays separated by the first birefringent plate;
the second Faraday element transmitting and emitting the other of the two polarized rays after the polarized rays swap the optical paths thereof and return from the reflecting device.
That is, each of the polarized rays separated by the first birefringent plate is transmitted once through the second Faraday element to give a predetermined rotating angle to each of the two planes of polarization. Since each polarized ray is transmitted only once through the second Faraday element, the plane of polarization undergoes a minimum influence of errors due to a product tolerance of the second Faraday element.
The first birefringent plate may be formed of any birefringent element. Preferred examples of such materials are rutile crystal expressed by TiO2, calcite expressed by CaCO3, lithium niobate expressed by LiNbO3 and yttrium vanadate expressed by YVO4. The first to third light transmitting devices, preferably, comprise single mode optical fibers, or plane of polarization maintaining optical fibers.
In a second aspect of this invention, a magnetic sensor utilizing a phenomenon in which a plane of polarization of light traveling through a Faraday element rotates in proportion to the strength of a magnetic field, the magnetic sensor comprising:
a light output device;
a sensor head connected to the light output device through a first light transmitting device;
a light detecting device connected to the sensor head through a second light transmitting device; and
a computing device for receiving detected signals from the light detecting device;
the sensor head including a first optical device, a first birefringent plate, a first Faraday element, a second birefringent plate and a third optical device arranged in series from an end of the sensor head connected to the first light transmitting device;
the light output device outputting light;
the first optical device of the sensor head converting polarized light transmitted from the light output device through the first light transmitting device into parallel light;
the first birefringent plate of the sensor head separating the parallel light received from the first optical device into two polarized rays having planes of polarization orthogonal to each other with respect to an optical axis of the birefringent plate;
the first Faraday element of the sensor head transmitting the two polarized rays from the first birefringent plate, and converting the strength of a magnetic field to be detected into a rotating angle of the planes of polarization of the two polarized rays;
the second birefringent plate of the sensor head separating each of the two polarized rays transmitted through the first Faraday element into two polarized rays (four polarized rays in total) orthogonal to each other with respect to an optical axis of the birefringent plate;
the third optical device of the sensor head selectively transmitting two orthogonal polarized rays returning along optical paths substantially the same as incidence optical paths, among the four polarized rays emitted from the second birefringent plate;
the light detecting device detecting light intensities of the two polarized rays transmitted from the second optical device through the second light transmitting device; and
the computing device deriving the strength of the magnetic field from the light intensities detected by the light detecting device.
Linear light outputted from the light output device may become elliptically polarized light indefinite in both axis and ellipticity due to disturbances occurring in the course of transmission to the sensor head. Such polarized light is passed through the first birefringent plate disposed in the sensor head, whereby the polarized light may be used as separated into two polarized rays orthogonal to each other based on the crystallographic axis of the birefringent plate, while retaining a total energy of light intensity, regardless of rotation of the planes of polarization. The two polarized rays have the planes of polarization rotated by magnetic field strength in the course of transmission through the first Faraday element. Thereafter the two polarized rays are transmitted through the second birefringent plate. Then, the two polarized rays are separated into two groups of two polarized rays (four polarized rays in total) orthogonal to each other according to a rotating angle of the planes of polarization, and are emitted to the second optical device. From the four polarized rays, the second optical device selects and outputs two polarized rays orthogonal to each other returning along optical paths substantially the same as incidence optical paths. The remaining two polarized rays are omitted by the second optical device. That is, the light intensities of the two polarized rays emitted from the optical device are variable with the rotating angle of the planes of polarization of the polarized rays emitted from the first Faraday element.
The light intensities of the two polarized rays are detected by the light detecting device, and are put to an arithmetic operation by the computing device, thereby obtaining detection results free from the influence of variations in the planes of polarization due to disturbances.
The above magnetic sensor in the second aspect of the invention may further comprise an element disposed between the first birefringent plate and the first Faraday element for giving a predetermined rotating angle to the plane of polarization of each of the two polarized rays separated by the first birefringent plate.
Alternatively, the magnetic sensor in the second aspect of the invention may further comprise an element disposed between the first Faraday element and the second optical device for giving a predetermined rotating angle to the plane of polarization of each of the two polarized rays separated by the first birefringent plate.
According to the above magnetic sensor, light that is to be detected without influences of a disturbance or magnetic field is passed through the second Faraday element disposed downstream of the birefringent plate or the first Faraday element, to apply a predetermined rotating angle to the planes of polarization beforehand. As a result, the light intensities detected by the light detecting device are varied according to the direction of the magnetic field to be measured.
That is, actual detection results may be compared with a reference provided by the light intensity detected when the magnetic field to be measured is xe2x80x9c0xe2x80x9d, whereby the direction as well as the strength of the magnetic field may be detected.
The magnetic sensor in the second aspect of the invention may further comprise an aperture plate disposed between the second birefringent plate and the optical device and defining a pinhole for passing the two polarized rays to be detected.
With the above construction, the pinhole in the plate disposed between the second birefringent plate and second optical device passes only two polarized rays to be detected. That is, where the first and second birefringent plates are formed thin to make the sensor head compact, the polarized rays transmitted through each birefringent plate have small refractive indexes (angles), making it difficult to select and output two polarized rays to be detected. However, by using the pinhole, only two polarized rays to be detected may be selected and outputted.
The first and second birefringent plates may be formed of any birefringent element. Preferred examples of such materials are rutile crystal expressed by TiO2, calcite expressed by CaCO3, lithium niobate expressed by LiNbO3 and yttrium vanadate expressed by YVO4. The first to third light transmitting devices, preferably, comprise single mode optical fibers, or plane of polarization maintaining optical fibers. Further, the element noted above, preferably, is a Faraday element having a predetermined magnetic field applied thereto beforehand.